1. Field of the Invention
The present invention relates to a time division multiple access digital transmission system comprising at least one primary station and a plurality of secondary stations, in which system the primary station transmits information to the secondary stations in frames comprising synchronisation information, and comprising user information in time slots, and in which system the secondary stations comprise a reference clock signal generator. Such a transmission system can be a mobile radio system, a cordless telephony system, or the like, or any system of the above kind in which primary and secondary stations have to be synchronised with respect to each other.
The present invention further relates to a secondary station for use in such a system.
2. Discussion of the Related Art
A time division multiple access digital transmission system of this kind is known from the handbook "The GSM System for Mobile Communications", M. Mouly et al, published by the authors, 1992, pp. 195-216, pp. 227-241, and from the handbook "Mobile Radio Communications", R. Steele, Pentech Press, 1992, pp. 696-698. In these handbooks a Time Division Multiple Access TDMA mobile radio system, or system arranged according to the so-called Global System for Mobile Communications (GSM standard), and sychronisation of a mobile radio station (MS) to a radio base station (BS) in such a GSM system is described. Although in general terms sychronisation is described, as according to the GSM Recommendation GSM 05.10, in which it is prescribed, for example, that the mobile radio station carrier frequency shall be accurate to within 0.1 parts per million, (PPM) or accurate to within 0.1 ppm compared to signals received from the radio base station, the GSM Recommendations do not specify the BS-MS synchronisation algorithms to be used, these being left to the equipment manufacturers.
In prior art GSM systems, synchronisation of a mobile radio station to a received datastream in time slots of TDMA frames may occur via two separate control loops. One control loop determines a time slot delay between a received time slot position intended for the mobile radio station and an expected time slot position. When an expected time slot position does not agree with the received time slot position, the expected time slot position is corrected with the difference between the two. Another control loop evaluates the frequency offset between a received carrier frequency and an expected received carrier frequency with respect to the frequency of a reference clock signal in the mobile radio station. The determined frequency offset is used to correct the frequency of the reference clock signal generator. A time slot sampling clock for an intermediate frequency or a base band signal derived from a received burst signal, and a data clock for clocking received data samples to a speech decoder, are derived from the reference clock signal.
Due to the fact that both, independently operating, control loops control are acting upon the same cause, i.e., a Doppler effect causing varying symbol delays in the transmission channel between the radio base station and the mobile station, and instability of the reference clock signal generator, the data clock may shift with respect to the time slot sampling clock. This leads to a data sample slip causing a discrepancy between a) reading in/out of an input-output-buffer for speech samples acquired from/to be submitted to signal bursts or time slots and b) the operation of a speech decoder/coder. Due to such a discrepancy, speech data may be lost. When other data than speech data, e.g., computer data or FAX data are exchanged between the radio base station and the mobile radio station, the data clock slip may even give rise to an unacceptable situation.
In another prior art synchronization solution the data clock is not derived from the reference clock signal, but is controlled separately. Unfortunately, such a solution necessitates more complex hardware.